This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Multiple genetic steps that result in the deregulation of two tumor suppressor pathways, governed by the p53 and retinoblastoma (Rb) tumor suppressors, pave the road to cancer in humans. The p53 and Rb proteins require communication between upstream effectors and activators to sense when a cell is under stress. Two proteins encoded by the INK4a/ARF locus, p16INK4a and p19ARF, functionally target the Rb and p53 tumor suppressors, respectively. These four proteins are among the most frequently affected genes in human cancer. We wish to understand the individual contribution of these proteins to the development of human cancers and how they may be regulated by upstream signals. We previously showed that ARF is induced by inappropriate mitogenic signals, such as those emanating from the Myc and Ras oncoproteins, and it diverts hyperproliferating cells to undergo p53-dependent cell cycle arrest or apoptosis. This is accomplished through ARF's interaction and nucleolar sequestration of the p53-negative regulator Mdm2. However, mounting evidence from our lab suggests that the ARF-p53-Mdm2 pathway is not be strictly linear, opening the door for further research into other ARF functions within the nucleolus. Our goal is to understand the basic mechanisms behind ARF's tumor suppressive capabilities and to relate these processes to our growing knowledge of human cancer progression. Additionally, ARF appears to antagonize ribosome processing as well as the nucleocytoplasmic shuttling of maturing ribosomal components.